US President Donald Trump has reignited his political feud with Sadiq Khan, calling him a "stone cold loser who should focus on crime in London".
Moments before Air Force One landed at Stansted, Donald Trump posted two tweets criticising the mayor of London.
It follows Mr Khan's attack on Mr Trump ahead of his three-day state visit to the UK.
A spokesman for Mr Khan said the "childish insults should be beneath the president of the United States".
....Kahn reminds me very much of our very dumb and incompetent Mayor of NYC, de Blasio, who has also done a terrible job - only half his height. In any event, I look forward to being a great friend to the United Kingdom, and am looking very much forward to my visit. Landing now!— Donald J. Trump (@realDonaldTrump) June 3, 2019
As he came in to land, Mr Trump wrote: "Sadiq Khan, who by all accounts has done a terrible job as Mayor of London, has been foolishly 'nasty' to the visiting President of the United States, by far the most important ally of the United Kingdom.
"He is a stone cold loser who should focus on crime in London, not me.
"Kahn [sic] reminds me very much of our very dumb and incompetent Mayor of NYC, de Blasio, who has also done a terrible job - only half his height.
"In any event, I look forward to being a great friend to the United Kingdom, and am looking very much forward to my visit. Landing now."
In response to Mr Trump's tweets, a spokesman for Mr Khan said: "This is much more serious than childish insults which should be beneath the president of the United States
"Sadiq is representing the progressive values of London and our country warning that Donald Trump is the most egregious example of growing far right threat around the globe."
New York City's Mayor Bill de Blasio later tweeted that he considered any comparison with London's mayor "a compliment".
A war of words (and blimps)
Mr Trump's tweets follow a long-running feud between the two men.
In May 2016 Mr Trump challenged the newly-elected London mayor to an IQ test after Mr Khan said his views on Islam were "ignorant".
Following the attack on London Bridge and Borough Market in 2017, the US president accused Mr Khan of "pathetic" behaviour.
Mr Khan responded that he would not allow Mr Trump to "divide our communities".
In July last year Mr Trump said Mr Khan had "done a very bad job on terrorism".
The mayor said he would not rise to Donald Trump's "beastly" accusation that he had done "a terrible job" following the London terror attacks.
Mr Trump's criticism came after Mr Khan permitted a plan to fly a giant inflatable "Trump baby" blimp to coincide with the president's UK visit.
Two months later Mr Khan also gave protesters permission to fly a bikini-clad blimp of himself over Westminster.
Mr Trump is taking part in his first official state visit to the UK as president.
It includes a private lunch with the Queen and a state banquet at Buckingham Palace.
Mr Trump will then meet Prime Minister Theresa May at St James's Palace on Tuesday morning for a business breakfast.
By BBC Assistant Political Editor Norman Smith
I would like to say I am surprised, but I am not because we know Trump has no regard for normal diplomatic niceties.
He seems to have got stuck in to Sadiq Khan. I am also not surprised because these two figures loathe each other.
This is a political grudge match which has been simmering now for three years, back from when the president introduced that travel ban on some Muslim countries.
Protests at Mr Trump's visit, including a "national demonstration" in Trafalgar Square, are planned for central London.
Both the Stop Trump Coalition and Stand Up to Trump protest groups said they would be present.
The Met Police said it had "a very experienced command team" leading the operation to deal with the visit.